Neuroendocrine Regulation Mechanism Of Dry Matter Intake Reducing And Fat Mobilization In Periparturient Dairy Cows

Leptin is a product of the ob gene, which is secreted from white adipocytes .It regulates food intake , energy balance , fat metabolism, endocrine, reproduction and immunological function. Leptin can affect hypothalamus that is a main target organ to implement it's function. Recent research indicated that leptin is a main signal of regulation of food intake and negative feedback loop of body weight in rodents and monogestric animal. Plasma liptin level raises up and effects feeding center in hypothalamic resulting in decrease of food intake and increases energy consumption, when body fat mass increases.;on the contrary,when the body fat mass decreases, the serum leptin tends to decline, and the sequel is that the food intake and energy export are regulated by the central pivot of neuroendocrine system. It is indicated that leptin is an important regulatory factor and has a very important role in body fat mobilization. NPY, a kind of peptide consisting of 36 amino acids, is considered as a superactive revulsive on food intake .It mainly resides in brain and regulates food intake, energy balance by controlling appetite and stimulating some endocrine factors. Recent researches indicated that NPY can promote food intake, and that the component that leptin legated with leptin receptor in the hypothalamus completes the regulation by way of inhibiting the biosynthesis and release of NPY. The metabolic feature in the peripartum dairy cow is decrease of the dry matter intake and negative balance of energy, causing fat mobilizations ,ketone body production ,and hepatic adiposum. Many factors can cause dry matter intake decrease in peripartum dairy cow .The regulation of ingestion and energy metabolism by metabolic signals is becoming a hot field. Metabolic signals include nutriments, metabolic products, reproductive hormones, stress hormones, leptin, insulin, insulin-like growth factor I (IGF-I),glucagons, cytokines, neuropeptide Y(NPY)etc. Among them leptin and NPY are the most important factors. Therefore, the relationships among the neuroendocrine factors, fat mobilization and the dry intake decrease in the postpartum cows is the key point. That establishes a theoretical foundation to elucidate the property of negative energy balance and pathogenesy of the metabolic diseases in peripartum dairy cow. In this study, cell culture in vitro, animal trial and quantitive RT-PCR were applied to research the effects of different energy intake levels in antepartum cows on dry matter intake (DMI), and expressing level of leptin receptor mRNA and hormone-sensitive lipase (HSL) mRNA in fat in postpartum cows. Thirty Chinese Hostein cows, with milk yield exceeding 5000Kg, were randomly allocated into three different energy level groups, GroupⅠof standard energy (control group,),GroupⅡof 120% standard energy and Group Ⅲof 80% standard energy from 28d before calving. After parturition, all the cows were offered standard diet to 56d. DMI, live weight, milk yield and its components, blood-biochemistry indices and endocrine factors relating to energy metabolism and food intake were measured. Blood and fat samples for determination of metabolites, hormone, and expressing level of hormone mRNA were obtained from w4ks prepartum to 4wks postpartum. The results showed that different levels of energy intake during antepartum canaffect DMI and lactating in the postpartum cows. The lower energy intake in antepartum dairy cows had the highst leptin and NPY concentrations in the serum, the fastest appetite and the most DMI .DMI in the cows increased rapidly and keep the high level during the period .The cows with high level of DMI reduced negative energy balance, and thus improved the dairy cow body conditions. On the contrary, The cows with high energy intake during antepartum had low concentrations of leptin and NPY in the serum , had slow recovery of appetite ,as well as the dry matter intake peak was postponed .As a result the energy negative balance in the cows was embarrassed and the cows had a bad body conditions. Dry matter intake and milk yield raised 3%～7% and 4%～8% in the low energy intake cows respectively. But, dry matter intake and milk yield declined 12%～17% and 10%～18% in the cows with higher energy intake, respectively. The low energy intake in antepartum dairy cows had 3%-7% DMI and 4%-8% milk more than control group, high intake had 12%-17% DMI and 10%-18% milk less than control group. The low energy intake in antepartum dairy cows displayed higher concentration of GLU, and less concentration of TG, ketone bodies, and NEFA in blood than those of control group of dairy cows, energy intake in antepartum dairy cows had lower concentration of GLU, and increased the concentration of TG, ketone bodies, and NEFA in blood than control group. It is thus evident that the high energy intake in antepartum dairy cows have more severe negative energy balance, and more mobilization fat. The result of real time PCR indicated that different levels of energy intake in antepartum affected the expressing level of leptin mRNA in fat of antepartum dairy cows such that the lower energy intake was obviously higher and the epressing level of HSL mRNA was obviously lower than those of the control group, and higher energy intake in antepartum dairy cows showed a reverse result, indicating that the high energy intake in antepartum dairy cows promoted fat mobilization. A positivecorrelation between leptin concentration in plasma and the quantity of adipose tissue leptin mRNA was observed. The dairy cow with higher leptin concentration in plasma had lower expressing level of HSL mRNA in fat. Leptin restrained the expressing of HSL mRNA, so that fat mobilization were decreased. It can be concluded that there is a relationship between neuroendocrine factors and fat mobilization. In order to observe the effects of leptin on the leptin receptors and triglyceride in vitro cell culture system, mesenteric cells from new born calf were sampled for separating and culturing. In this culture model ,different concentration of leptin was added ,the cell growth was recorded, leptin mRNA,HSL mRNA and Ob-Rb mRNA in the cultured cells in vitro were determined by half quantitative RT-PCR . The results indicated that recombinant leptin could promote the lipolysis of fat cells with decreasing the size of cells and content of triglyceride in the mature fat cells. The expressing level of leptin mRNA in adipose cell was declined followed by enhancing leptin concentration in culture medium. Leptin increased the expressing level of HSL mRNA and≤5ng/ml of leptin can enhance the expressing of long style receptor (ob-rb) mRNA in new born calf adipose cell for 12 to 24h culture. On the whole, it is concluded that energy intake level during antepartum impacts dry matter intake in postpartum in dairy cows and regulats expressing of the key enzyme of fat mobilization based on the data in vitro and in vivo experiments. The dry matter intake obviously decreases in dairy cows with high energy intake during antepartum, aggravates negative energy balance, causing ketosis and fatty liver. High energy intake during antepartum obviously decrease dairy dry matter in the cows, possibly resulting from reducing in synthesis and secretion of NPY. The cows with high energy intake during antepartum have a severe negative energy balance, lost the most body weight and mobilize the most fat,possibly resulting from down-regulation HSL expression in fat cells by leptin. Therefore, the reasons of declining dry matterm...